ARINC 429
Data bus for Civil Aircraft
Prepared
by
A.Kaviyarasu
Assistant Professor
Department of Aerospace Engineering
Madras Institute Of Technology
Chromepet, Chennai
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This presentation is about ARINC 429 Data bus which is commonly used in all
civil aircrafts.
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ARINC- stands for “Aeronautical Radio INC”
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ARINC is a private corporation organized in 1929 comprises of a airlines,
aircraft manufacturers and avionics equipment manufacturers to produce
specifications and standards for avionics equipment.
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ARINC 429 is a standard for requirements for transfer of digital data between
avionics systems on commercial aircraft.
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ARINC 429 is also known as the Mark 33 DITS (Digital Information Transfer
system) Specification.
Typical photographic of ARINC 429
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ARINC 429 uses simplex, twisted shielded pair data bus.
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It consist of one transmitter and multiple receiver (up to 20 receiver connected to one
twisted pair of wire).
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It does not have Bus monitor, Remote terminal or Bus controller as like that of 1553B
data Bus.
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A transmitter may „talk only‟ to a number of receivers on the bus, up to 20 on one wire
pair, with each receiver continually monitoring for its applicable data, but does not
acknowledge receipt of the data.
•
A transmitter may require acknowledgement from a receiver when large amounts of data
have been transferred. This handshaking is performed using a particular word style, as
opposed to a hard wired handshake. When this two way communication format is
required, two twisted pairs constituting two channels are necessary to carry information
back and forth, one for each direction.
• Communication on 429 data bus uses 32 bit words with odd parity. The
wave form uses is bipolar return to zero with each bit lasting either 70 or 83
microsec+_2.5 percent or 10microsec+_2.5 percent depends on operating
speed of bus.
• ARINC-429 data bus operate in two speeds
– Low speed (12.5 kHz )
– High speed (100kHz)
• Low speed bus is used for general purpose low critical application .
• High speed bus is used for transmitting large quantities of data's or flight
critical applications.
• Transmission from the source LRU is comprised of 32 bit words
containing a 24 bit data portion containing the actual information,
and an 8 bit label describing the data itself.
• LRUs have no address assigned through ARINC 429, but rather
have Equipment ID numbers which allow grouping equipment into
systems, which facilitates system management and file transfers.
• Sequential words are separated by at least 4 bit times of null or zero
voltage.
• Transmission rates may be at either a low speed – 12.5 kHz – or a
high speed – 100kHz.
Star Topology
Receiving
LRU
Receiving
LRU
Receiving
LRU
Transmitting
LRU
Receiving
LRU
Receiving
LRU
Receiving
LRU
Bus Drop Topology
Transmitting
LRU
Receiving
LRU
Receiving
LRU
Receiving
LRU
Receiving
LRU
Multiple Bus Design Topology
Tx
Rx
Rx
Tx
LRU
RX
RX
Tx
LRU
RX
RX
RX
RX
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ARINC 429 data words are 32 bit words made up of five primary fields:
Parity – 1 bit
Sign/Status Matrix (SSM) – 2 bits
Data – 19 bits
Source/Destination Identifier (SDI) – 2 bits
Label – 8 bits
Parity BIT
• Parity bit helps to Detect Error in the receiving words.
• ARINC 429 uses odd parity as error check to insure accurate
data reception.
• The number of Logic 1s transmitted in each word is an odd
number. The receiver checks the received words having odd
logic or not.
• Odd logic 1s - accept the word
• Even logic 1s- reject the word
ARINC 429 data types
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BNR Word Format (Signed Binary Number)
– Encoded and transmitted by using 2s complement fractional binary notation
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BCD Word Format (Binary coded Decimal)
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Discrete Data-combination of Both BNR and BCD
– Combination of BNR, BCD or individual bit representation
•
Maintenance data
communication
and
Acknowledgement
Data-Requires
two
way
– Alphanumeric Data encoded
•
File data transfers by using Williamsburg/Buckhorn Protocol – A bit-oriented
protocol.
ARINC Bnr word Format
• It represent the entire data as Binary Number using 2s complement method.
• BIT 29 is used to represent as a signed number.
• Bit 29 with 1 indicating a negative number – or South, West, Left, From or
Below.
• Bit 28 is MSB used to represent half the maximum value of data.
• Bit 27 is half the value of Bit 28.
• Bit 26 is half the value of Bit 27.
ARINC BCD word Format
• It uses 4 data field bits to represent each decimal digit.
• 5 subfields are used to represent decimal value. Among 5 data
fields. MSB Digit 1 only have 3 data field bits (for maximum
binary value of 7).IF MSB digit is greater than 7 bit 29-17 are
padded with zero and second subfield become MSB allowing
4 data field instead of 5data field to represent the data.
Discrete Data word Format
• Discrete data can be made up of BNR and/or BCD data,
or as individual bits representing specific equipment
conditions. Pass/Fail, Activated/Non-Activated and
True/False conditions relating to system or subsystem
operational activity can be represented by setting or
clearing predefined bits in the word data field.
Maintenance Data and Acknowledgement
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It can only employ or access by using Duplex or Two way communication
between source and the sink (receiver).
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Since ARINC 429 only uses one way simplex transmission.
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Two ARINC channels are required for an LRU to send and receive
maintenance, Acknowledgement and sequence of messages.
Williamsburg/Buckhorn Protocol
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Williamsburg/Buckhorn is a bit oriented protocol used to File Transfer across an
ARINC bus.
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It is updated version of AIM data (Acknowledge, ISO Alphabet NO.5 and
Maintenance Data) which was used in earlier version of ARINC 429.
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File transfer are necessary when more than 21 bits of data are required to
transmitted.
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When initiating a file transfer – the source and receiver need to establish first.
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Followed by source initiate a predefined code to determine compatibility with
receiver units.
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A request to send word (RTS) is Transmitted containing a destination code and
word count.
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The receiver responds with a Clear to send word (CTS) retransmitting back to the
destination and word count information.
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Upon confirmation of CTS, the source begins the file transfer.
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Files are transferred in Link Data Units (LDUs) format. Means, the entire file is
divided into 3 to 255 words of length depends upon size of the file.
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File transfer begin with a Start of Transmission word (SOT) which includes file
sequence number, general format identifier(GFI), and LDU sequence Number.
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After the SOT ,the data words are sent, followed by end of transmission (EOT).
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The EOT includes a CRC and LDU position in the file being sent. The receiver
decodes the files depends upon their LDU position.
Sign/Status Matrix
• BIT 31-30 are assigned for Sign/Status Matrix or SSM
• Depending upon the word labels(1-8 bit),the SSM provides
Different information about status of subsystem and status of
the word.
•
For BCD ,the SSM can be used to indicate Sign or Direction
of the data contained in the ARINC word.
SSM for BCD word
BIT 31
BIT 30
Decoded Information
0
0
Plus, North, East, Right, To Above
0
1
No Computed Data
1
0
Functional Test
1
1
Minus, South, West, Left, From, Below
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Use of sign function for BCD is optional
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NO Computed Data Code (01) is used to identify a source system that is not
able to produce reliable data
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Functional code is transmitted along with an Instruction command code to
perform a test on receiver unit. The data received back from the receiver
contains result of the test
SSM for BNR word
BIT 31
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BIT 30
Decoded Information
0
0
Failure warning
0
1
No Computed Data
1
0
Functional Test
1
1
Normal Operation
BIT 29
Decoded Information
0
Plus, North, East, Right, To Above
1
Minus, South, West, Left, From, Below
Failure warning code (00) indicates a transmitter or source system is failure that could
not produce an reliable data.
NO Computed Data Code (01) is used to indicate unreliable data output by a condition
other than system failure.
Functional code (10)- indicates the word data containing results of the Functional test.
Bit 29 is optional in BNR word Format.
SSM for Discrete word
BIT 31
BIT 30
Decoded Information
0
0
Verified Data, normal operation
0
1
No Computed Data
1
0
Functional Test
1
1
Failure warning
• Use of sign function for BCD is optional.
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NO Computed Data Code (01) is used to identify a source system that
is not able to produce reliable data.
• Functional code is transmitted along with an Instruction command code
to perform a test on receiver unit. The data received back from the
receiver contains result of the test.
Source/DestiNation identifier
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The source/Destination identifier (SDI) utilizes the bit 9-10.Its is optional bit
used to identify the source which is transmitting the data or multiple receiver to
identify which receiver the data is meant for.
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For Higher resolution file transmission the BIT 9 and 10 may be used instead
of SDI.
Label
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BIT 1-8 contains the ARINC Label also known as Information Identifier.
The label is expressed as a 3 Digit octal number.
The label is used to identify the word data type whether it is BNR, BCD,
Discrete word and also contains instruction or Data reporting information.
Label may be extend by utilizing the Bits 11-13, as an Equipment Identifier to
identify the bus transmission source. Equipment IDs are expressed in
hexadecimal values.
Cable Characteristic
• The transmission bus media uses 78 Ω shielded twisted pair cable.
The shield must be grounded at each end as well as all junction along
the bus.
• The transmitting source output impedance should be 75 Ω ± 5 Ω
divided equally between Line A and Line B. This balanced output
should closely match the impedance of the cable.
• The receiving sink must have an effective input impedance of 8kΩ
minimum.
• Maximum length is not specified, as it is dependent on the
number of sink receivers, sink drain and source power.
• Most systems are designed for under 150 feet, but conditions
permitting, can extend to 300 feet and beyond.
Transmission characteristic
• Data is transmitted in a
bipolar, Return-to-Zero
format.
• This is a tri-state modulation
consisting of HIGH, NULL
and LOW states.
• Voltages presented across the
receiver input will be
dependent on line length,
stub configuration and the
number of receivers
connected.
• Transmission voltages are
measured across the output
terminals of the source
Example ARINC 429 Data format